Display devices such as television sets and movie projectors often incorporate a modulator for the purpose of distributing light into a two-dimensional pattern or image. For example, the frames of a movie reel modulate white light from a projector lamp into shapes and colors that form an image on a movie screen. In modern displays light modulators are used to turn on and off individual pixels in an image in response to electronic signals that control the modulator.
Texas Instruments introduced a microelectromechanical, integrated circuit chip, light modulator called a digital mirror device which includes millions of tiny mirrors on its surface. Each mirror corresponds to a pixel in an image and electronic signals in the chip cause the mirrors to move and reflect light in different directions to form bright or dark pixels. See, for example, U.S. Pat. No. 4,710,732 incorporated herein by reference. Stanford University and Silicon Light Machines developed a microelectromechanical chip called a grating light modulator in which diffraction gratings can be turned on and off to diffract light into bright or dark pixels. See, for example, U.S. Pat. No. 5,311,360 incorporated herein by reference.
Both of these reflective and diffractive light modulation schemes for displays involve two-dimensional arrays of light modulator elements. However, it is also possible to make a display in which light is incident on a linear array of high speed light modulators. With appropriate magnifying optics and scanning mirrors, a linear array can be made to appear two-dimensional to an observer. Through the scanning action of a vibrating mirror a single row of light modulators can be made to do the work of as many rows of modulators as would be necessary to provide a real two-dimensional display of the same resolution. See, for example, U.S. Pat. No. 5,982,553 incorporated herein by reference.
Manhart introduced a display apparatus including a grating light-valve array and interferometric optical system. See U.S. Pat. No. 6,088,102 incorporated herein by reference. In Manhart a display system employs a planar grating light-valve (GLV) array as a spatial light modulator for representing an image to be displayed. The system relies for image representation on the position of moveable reflective elements of the GLV array, which move through planes parallel to the plane of the array. The moveable elements provide, from an incident phase-contrast wavefront, a reflected phase-modulated wavefront representing the image to be displayed. The displayed image is provided by interferometrically combining the phase-modulated wavefront with a reference wave-front also formed, directly or indirectly, from the incident phase-contrast wavefront.
Many microelectromechanical light modulators are compatible with digital imaging techniques. Digital information may be sent electronically to the modulator. For example, gray scale images may be achieved by turning pixels on only part time. A pixel that is switched from bright to dark with a 50% duty cycle will appear to an observer to have a constant intensity half way between bright and dark. However, the pixel must be switched between bright or dark states faster than the human eye's critical flicker frequency of roughly 30 Hz or else it will appear to flicker. Therefore two-dimensional digital light modulators for displays must switch between states quickly to display a range of light levels between bright and dark.
A one-dimensional digital light modulator array, scanned by a vibrating mirror to make it appear two-dimensional, must incorporate modulators with fast switching speeds. Each modulator element must switch on and off quickly to provide the impression of gray scale and this action must be repeated for each pixel in a line within the scanning period of the mirror. Grating light modulator devices in particular exhibit high switching speeds because their mechanical elements move only very short distances. The grating light modulator incorporates parallel ribbon structures in which alternating ribbons are deflected electrostatically to form diffraction gratings. The ribbons need only move a distance of one quarter wavelength of light to switch a grating on or off. It is also possible (and desirable in many instances) to operate one- or two-dimensional light modulators in analog, rather than digital, modes.
One limitation of the grating light modulator is that at least two ribbons are required in order to form a diffractive modulator element. Therefore each pixel requires at least two ribbons each of which uses up valuable space on a chip. Another limitation of grating light modulators is that they require collimated light sources. Gudeman proposed an interferometric light modulator based on a mechanical structure very similar to the grating light modulator; see U.S. Pat. No. 6,466,354 incorporated herein by reference. Gudeman's light modulator is a form of Fabry-Perot interferometer based on a ribbon structure.
Microelectromechanical light modulators typified by the Texas Instruments' digital mirror device and Stanford/Silicon Light Machines grating light modulator devices mentioned above have already enjoyed wide commercial success and have spawned other related designs. See, for example, U.S. Pat. No. 6,724,515 incorporated herein by reference. However, they are not without limitations and there is room for improvement.
The digital mirror device is comparatively slow and therefore is usually supplied as a two-dimensional mirror array. Usually two dimensional modulator arrays are more expensive to make than one-dimensional arrays and require a sophisticated addressing scheme for the mirrors. A two-dimensional array requires defect-free manufacturing of N×N pixels over a large chip area while a one-dimensional array with the same image resolution requires only N working pixels on a chip in a single line.
Grating light modulator devices, while very fast, require more than one ribbon structure per pixel as noted above. They are also affected by limitations due to diffraction. A grating light modulator has a reflective state or configuration and a diffractive state. In the diffractive state incoming light is diffracted into the +1 and −1 diffraction orders of an optical grating. However, only about 80% of the light is collected in these two orders. Light diffracted into higher orders is lost and overall light efficiency suffers.
Grating-based devices use high numerical aperture optical elements to collect diffracted light. It would be desirable for a modulator to be able to use simpler, low numerical aperture optics. Grating-based devices also have some difficulty achieving high contrast in the dark state; i.e. displaying black areas in an image. A light modulator that escaped as many of the limitations of existing modulator designs as possible would be highly desirable.
An interferometric light modulator that has many desirable features was disclosed in “Differential interferometric light modulator and image display device,” U.S. Ser. No. 10/904,766 filed on Nov. 26, 2004, incorporated herein by reference. That device features high speed, high contrast and is compatible with relatively low numerical aperture optics. Additionally each pixel in the displayed image depends on only one MEMS ribbon (or other phase shifting structure) so valuable chip real estate is saved. The interferometric design means that light is not lost in higher diffractive orders as can be a problem in diffractive devices.
Further improvement is always possible, however. The simplest improvement to understand conceptually would be a design that lends itself to compact construction. More subtle advances might involve different ways of manipulating polarized light and improvement of optical parameters such as field-of-view. Additionally variations on MEMS devices for imparting optical phase shifts are desirable in different applications.